NK cell development is a continuum from the most primitive hematopoietic stem cell to the killer immunoglobulin-like (KIR) positive, terminally differentiated cell. The role of weakly cytotoxic, KIR negative, interferon-gamma producing CD56[+bright] cell in this continuum is unknown. Little is known about how and when receptor fate is determined in this developmental process. We have made significant progress during previous years if this R01. We now have a robust system, which allows us to separate early and late events in NK cell differentiation. Using this system, although it is well established that IL-15 plays a critical role in NK cell development, our current data suggest that the earliest stages of NK cell differentiation may be IL-15 independent. The overall theme of the current proposal is to test the hypothesis that mechanisms of KIR and NKG2 receptor acquisition are determined at discrete developmental stages of NK cell maturation. Our current progress suggests that several manipulations impact on KIR expression including IL-3 signaling, possibly through STAT5 pathways. We hypothesize that 1) KIR acquisition signals are mediated in an NK cell precursor (defined as CD56 and CD16 negative, no cytokine production, no cytotoxic activity), 2) IL-3 induces a transcription factor in NK precursors which serves as a switch for KIR acquisition, 3) encountering class. I MHC during development plays a role to influence the final KIR repertoire and 4) CD56[+bright] NK cells represent a distinct NK cell lineage rather then a sequential intermediate giving rise to the terminally differentiated CD56[+dim] NK cell. The significance of these studies is of great translational interest given the recent flurry of publications suggesting a role for NK cells and their receptors in allogeneic transplant clinical outcomes (leukemia relapse and graft versus host disease). The investigations are divided into the following three related specific aims. Specific Aim 1: NK Cell development from the hematopoietic stem cell. Specific Aim 2: Mechanisms of KIR acquisition in developing NK cells. Specific Aim 3: The lineage relationship between CD56[+bright] and CD56[+dim] NK cells